In an electrophotographic copier, an original image is exposed to light. The reflected light is irradiating a photosensitive drum or belt to form an electrostatic latent image thereon.
In an electrophotographic printing machine, a photoconductive medium is image-wise exposed by a LED, LED-array or scanning laser for forming an electrostatic latent image.
Toner is deposited on the latent image, wherein a toner image is formed on the drum or belt. This image is transferred onto a receiving sheet or web by a transfer unit and is fixed onto the receiving medium by a fixing or fusing unit.
Direct Electrostatic Printing is performed directly on a substrate by means of electronically addressable printheads. In Direct Electrostatic Printing the toner or developing material is deposited directly in an image-wise way on a substrate. The substrate can be an intermediate but it is preferentially the final receptor after a final fusing step.
In a fusing method utilizing thermal energy, a toner image formed on the receiving medium is melted by heating so as to adhere to the sheet. For this purpose the toner image is generally pressed by a roller heated-up to the temperature at which the toner material becomes adhesive.
In the roller fixing or fusing unit, a heater provided inside a roller is switched on/off under control of the temperature control system of the roller by means of a temperature detecting element provided near the surface of the roller.
A fixing or fusing roller is usually composed of a cylindrical metallic core preferably of aluminium, coated with silicone rubber or fluoroelastomer, or silicone rubber with a fluororesin coating, in order to obtain a proper removability property of toner particles.
Silicone rubber has a low thermal conductivity and therefore, the surface temperature of the fixing roller largely varies with the passing-through of the recording sheets. Temperature variations of the fixing roller result in image degradations such as gloss variations and colour instabilities.
In order to solve the above problems, various countermeasures have been taken: U.S. Pat. No. 5,504,567 discloses a temperature control with feedforward. A Schmitt-predictor is another method for refinement of temperature control as known by those skilled in the art of system control theory.
U.S. Pat. No. 5,493,378 discloses a method to remedy the difference between first and second roller revolutions, leading to visible marks on the image, especially for heavy paper, by using two different fusing speeds.
Most electrophotographic apparatuses have an upper limit in thickness of the receiving media they can handle. A heated fixing roller that has been kept at a standby temperature loses heat as it fuses the toner images, thereby lowering its temperature. Although the temperature control of the fuser immediately begins to compensate for the lowered temperature by increasing the electrical power to its heaters, the immediate temperature drop changes the heat actually applied to the image. The total heat imparted to the toner image controls the amount of gloss of the image. Irrespective of the gloss amount of the image, the gloss evenness across the image is most important.
Preferred gloss levels for xerographic colour images on various paper types are described in: E. N. DALAL and P. C. SWANTON "Electronic Imaging" Vol.5,nr 2 "Preferred gloss levels for colour images". This article describes that a lower image gloss is preferred for the business graphics images than for the pictorial images on a given paper.
U.S. Pat. No. 5,300,995 discloses a heated pressure roller fuser which gradually reduces the speed of the rollers in order to compensate for the temperature drop caused by the loss of heat to the sheet.